In recent years, reduction in energy consumption has been considered a major technical issue for electrophotographic devices. Another issue for electrophotographic devices is to significantly reduce the heat output of fixing devices.
By lowering the toner fixing temperature, it is possible to both save energy for the electrophotographic device while also allowing a wider range of toner anchoring conditions. In addition, this can shorten the latency time upon power activation of the electrophotographic device, required until the temperature of the surface of the fixing members such as fixing rolls reaches a temperature that allows fixing, known as the warm-up time, and can extend the life of the fixing member. However, lowering the toner fixing temperature also simultaneously lowers the glass transition point of the toner particles. It is therefore difficult to simultaneously achieve higher storability (blocking resistance) for toners. In order to achieve both toner fixability at low temperatures (a low-temperature fixability) and storability of the toner, it is necessary for the glass transition point of the toner to be maintained at higher temperatures. In addition, toner must have a “sharp melt property”, whereby the viscosity of the toner rapidly falls in the high-temperature range.
For this reason, toners have been introduced that employ crystalline polyester resins. Crystalline polyester resins do not exhibit a clear glass transition temperature due to the regular arrangement of the molecular chains. Therefore, crystalline polyester resins have the property of being resistant to softening up to the crystal melting point. Much attention has therefore been directed toward materials that have both storability and a low-temperature fixability.
Methods of using crystalline polyester resins as binder resins have long been known (see PTL 1 and PTL 2, for example). However, crystalline polyester resins are difficult to pulverize by kneading pulverization methods, and their yields are low. Crystalline polyester resins therefore tend to be poorly practical from the viewpoint of productivity. If practical productivity can be achieved, this would allow the fixing temperature of the toner to be lowered. However, it is not always possible to obtain sufficient hot offset resistance. Because fused toner penetrates too deeply into paper, this tends to prevent high-density images from being obtained. In addition, crystalline polyester resins tend not to exhibit the electrical resistance necessary for electrification, due to the structures of the resins themselves. Consequently, crystalline polyester resins are generally studied for their use in combination with amorphous polyester resins.
The invention described in PTL 3 provides an electrostatic charge image developing toner having a low-temperature fixability as well as sufficient electrostatic properties. PTL 3 discloses an electrostatic charge image developing toner comprising at least a crystalline polyester resin and an amorphous polyester resin as binder resins, and having its surface covered with a surface layer composed mainly of an amorphous polyester resin, wherein the crystalline polyester resin content is in the range of 30 to 80 wt %, the proportion of crystalline polyester in the outermost surface of the electrostatic charge image developing toner is no greater than 15 atomic %, and the mean thickness of the surface layer is between 0.01 μm and 0.5 μm.
The invention described in PTL 4 relates to a crystalline polyester resin composed mainly of a C2-12 aliphatic diol and a C8-12 aliphatic dicarboxylic acid compound, and is obtained by condensation polymerization with a molar ratio (carboxylic acid component/alcohol component) of 1.03 to 1.20. According to the invention described in PTL 4, improved storability and environmental stability are obtained while maintaining the low-temperature fixability, by controlling the low-molecular-weight component which is a carboxyl group at the end of the crystalline polyester resin.